1. Field of the Invention
This invention relates generally to the field of optical fiber communications. More specifically, the invention relates to the use of frequency-division multiplexing (FDM) in optical fiber communications systems.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3 and OC-3 connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the OC protocol have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal.
One approach to address the increasing demand for communications bandwidth is to simply add more optical fiber to the current networking infrastructure. However, this is not always a viable alternative. There are many areas of the country, for example metropolitan areas, where the ducts or conduits carrying optical fiber are filled to capacity or where the fiber was originally buried directly into the ground. In addition, adding more fiber is often both prohibitively expensive and time-consuming, due to high installation costs and local resistance to the disruption caused by fiber installation. These factors therefore favor solutions which increase communications bandwidth by more efficiently utilizing the installed fiber infrastructure rather than by installing new fiber.
Furthermore, other factors also favor solutions other than installing new fiber. For example, because of its large inherent bandwidth, an optical fiber is most efficiently used when multiple users share the fiber. Typically, a number of low-speed data streams (i.e., xe2x80x9clow-speed channelsxe2x80x9d), for example transmitted by different users, are combined into a single high-speed channel for transport across the fiber. Conversely, when the high-speed channel reaches the destination for one of the low-speed channels contained in it, the low-speed channel must be extracted from the rest of the high-speed channel. A typical optical network consists of nodes which transmit high-speed channels to each other over optical fibers. In addition to transporting low-speed channels through the node (the xe2x80x9cpass-throughxe2x80x9d function) as part of high-speed channels passing through the node, nodes may also combine incoming low-speed channels to the high-speed channel (the xe2x80x9caddxe2x80x9d function) and/or extract outgoing low-speed channels from the high-speed channels (the xe2x80x9cdropxe2x80x9d function). These functions are commonly referred to as add-drop multiplexing (ADM).
Increasing the ADM functionality of nodes in a network increases the flexibility of the network, thus increasing the number of applications and network configurations that may be implemented by the network. For example, metropolitan networks are characterized by densely populated areas, a large number of nodes (e.g., central offices), short distances between nodes (typically less than 40 km), and lower data rates than long distance networks (typically less than 2.5 Gbps). The traffic patterns for metropolitan networks change rapidly and require dynamic interconnections at the large number of nodes, which are often remotely managed. ADM functionality allows low-speed channels to be remotely added to or dropped from a high-speed channel, thus addressing the requirements of the metropolitan network.
However, the manner in which the ADM functionality is implemented in a particular network will depend in part on how the low-speed channels are combined to form a high-speed channel. Thus, an approach which addresses the capacity problem by combining a large number of low-speed channels into a high-speed channel may not be favored if it does not readily support ADM functionality. A good approach should both increase the number of low-speed channels contained in each high-speed channel and also support significant ADM functionality.
Two widely used approaches to combining low-speed channels are wavelength division multiplexing (WDM) and time division multiplexing (TDM). In WDM or its more recent counterpart dense wavelength division multiplexing (DWDM), each low-speed channel is placed on an optical carrier of a different wavelength and the different wavelength carriers are combined to form the high-speed channel. Crosstalk between the low-speed channels is a major concern in WDM and, as a result, the wavelengths for the optical carriers must be spaced far enough apart (typically 50 GHz or more) so that the different low-speed channels are resolvable. In TDM, each low-speed channel is compressed into a certain time slot and the time slots are then combined on a time basis to form the high-speed channel. For example, in a certain period of time, the high-speed channel may be capable of transmitting 10 bits while each low-speed channel may only be capable of transmitting 1 bit. In this case, the first bit of the high-speed channel may be allocated to low-speed channel 1, the second bit to low-speed channel 2, and so on, thus forming a high-speed channel containing 10 low-speed channels. TDM requires precise synchronization of the different channels on a bit-by-bit basis (or byte-by-byte basis, in the case of SONET), and a memory buffer is typically also required to temporarily store data from the low-speed channels.
In the case of WDM, one approach is to implement the ADM functionality entirely in the optical domain. This avoids having to convert the high-speed channel from optical to electrical form, but has a number of other significant limitations. First, as described previously, the wavelengths for each of the optical carriers in a WDM system typically are spaced far apart (e.g. 50 GHz or more). As a result, the number of different optical carriers is limited and if each carrier corresponds to a low-speed channel, as is typically the case, the total number of low-speed channels is also limited. Furthermore, if the bandwidth capacity of the fiber is to be used efficiently, each low-speed channel must have a relatively high data rate due to the low number of low-speed channels, thus preventing add-drop at a fine granularity. For example, if the high-speed channel has a total capacity of 10 Gigabits per second (10 Gbps) and is allotted a bandwidth of 200 GHz, then current WDM systems will typically be limited to no more than four low-speed channels, each of which will be 2.5 Gbps in order to meet the overall bit rate of the high-speed channel. However, this means that the low-speed channels can only be added or dropped in blocks of 2.5 Gbps. Since many data streams occur at a much lower bit rate, such as at 155 Megabits per second (Mbps) for OC-3, it is often desirable to add and drop at a granularity which is finer than what WDM can support.
The current state of technology also limits the practicality of all-optical ADM. In all-optical approaches, the channels typically are not regenerated as they pass through each node in the network and will continuously deteriorate until they reach their final destination. As a result, the entire network must be designed assuming deterioration along the worst-case path through the network. In contrast, if a channel is regenerated at each node, the network may be designed based only on node-to-node deterioration, regardless of the total number of nodes in the network. As another example, current technology makes it difficult to route a low-speed channel occupying one wavelength of a high-speed channel to a different wavelength of the high-speed channel. This severely limits the ADM functionality that may be implemented since low-speed channels are not freely routable. For example, if a low-speed channel occupies a particular wavelength on an incoming high-speed channel, that low-speed channel can only be passed through to another high-speed channel if that particular wavelength on that high-speed channel is unoccupied, regardless of how many other wavelengths are available.
An alternate approach to implementing ADM functionality for WDM systems is based on converting the optical high-speed channels to electrical form and then performing the ADM function electrically. This approach, however, is expensive since it requires significant amounts of both optical and electrical devices. WDM is an inherently optical approach and requires optical devices to implement. On the other hand, an electrical ADM would require significant electrical devices to implement. Combining the two would require both sets of devices and would additionally require optical-to-electrical (O/E) and electrical-to-optical (E/O) converters, typically one set for each wavelength used in the WDM.
As a result of the disadvantages described above, ADM capabilities in current WDM systems are often fixed or limited. For example, add/drop connections between low-speed channels and high-speed channels may be fixed when a node is installed and may be changed only by a corresponding change in hardware. As another example, the add/drop functions may be implemented only for a subset of the low-speed channels connected to a node. Alternately, a node may be able to implement only a subset of all possible connections between low-speed channels and high-speed channels. These compromises reduce the overall ADM functionality of the node and its flexibility within a network.
Implementing ADM capabilities for TDM networks also has significant disadvantages. First, as mentioned above, the TDM approach is strongly time-based and requires precise synchronization of the channels entering and exiting the ADM to a common reference clock. As a result, TDM systems require significantly more complex timing recovery, leading to increased overall cost. In addition, since the low-speed channels typically are combined on a bit-by-bit (or byte-by-byte) basis, TDM systems are heavily dependent on the bit rates of the individual low-speed channels and have difficulty handling low-speed channels of different bit rates or different protocols. As yet another disadvantage, TDM systems generally require significant amounts of buffer memory since bits from the low-speed channels typically must be temporarily stored before they can be properly sorted and time-synchronized to form a high-speed channel. These required buffers add to the cost of implementing an ADM within a TDM system.
Thus, there is a need for an inexpensive node which efficiently combines a number of low-speed channels into a high-speed channel and which also provides a broad range of ADM capabilities for optical communications networks, in particular including the functionalities of adding, dropping, drop-and-continue, and pass-through of a low-speed channel. The node preferably implements the ADM functionalities independent of bit rate, format, and protocol of the various channels and is capable of handling a large number of fine granularity low-speed channels. There is further a need for a node which regenerates the channels passing through it.
In accordance with the present invention, an FDM node for use in optical communications networks includes an O/E converter, a frequency division demultiplexer, an E/O converter, a frequency division multiplexer, and an electrical ADM crosspoint. In the high-speed receive direction, the O/E converter converts a first optical high-speed channel to a first electrical high-speed channel. The frequency division demultiplexer is coupled to the O/E converter and frequency division demultiplexes the first electrical high-speed channel into a first plurality of low-speed channels (preferably at the same data rate as STS-3 signals) which are transmitted to inputs of the ADM crosspoint. In the transmit direction, the frequency division multiplexer receives a second plurality of low-speed channels from outputs of the ADM crosspoint and frequency division multiplexes them into a second electrical high-speed channel, which is then converted by the E/O converter to a second optical high-speed channel. The ADM crosspoint switchably couples its inputs to its outputs, thereby implementing an add/drop multiplexing (ADM) function for the optical high-speed channels.
In another aspect of the invention, the transmit side of an FDM node for use in optical communications networks includes a quadrature amplitude modulation (QAM) modulator, a frequency division multiplexer, and an E/O converter coupled in series. The term QAM is to be interpreted in its most general sense, with multiple signal phases and multiple signal amplitudes. As such, it includes common constellations such as BPSK, QPSK, 8PSK, 16-QAM, 32-Cross, 64-QAM, etc, as well as arbitrary complex constellations. The QAM modulator applies QAM is modulation to the low-speed channels to form FDM channels. The low-speed channels preferably are characterized by data rates greater than 100 million bits per second and forward error correction codes may also be applied. The frequency division multiplexer converts the FDM channels into an electrical high-speed channel, preferably using a two stage IF/RF process. The E/O converter converts the electrical high-speed channel to an optical high-speed channel.
In yet another aspect of the invention, the corresponding receive side includes an O/E converter, a frequency division demultiplexer, and a QAM demodulator coupled in series. The O/E converter converts an optical high-speed channel to an electrical high-speed channel. The frequency division demultiplexer separates the electrical high-speed channel into its constituent FDM channels. The QAM demodulator demodulates the FDM channels into the original low-speed channels.
In another aspect of the invention, a method for transporting data includes the following steps. A first optical high-speed channel is received and converted to a first electrical high-speed channel. This is frequency division demultiplexed into a plurality of first low-speed channels, which are to be passed-through to a second optical high-speed channel. The first low-speed channels are switchably coupled to second low-speed channels. These are frequency division multiplexed to produce a second electrical high-speed channel, which is converted to the second optical high-speed channel.
The FDM-based approach is particularly advantageous because the use of frequency division multiplexing results in the efficient combination of low-speed channels into a high-speed channel and the efficient separation of a high-speed channel into its constituent low-speed channels. For example, since the multiplexing occurs in the electrical domain rather than the optical one, this approach requires only a single optical to electrical conversion (e.g., the optical high-speed channel into an electrical high-speed channel), whereas approaches like WDM would require multiple optical to electrical conversions (e.g., one for each wavelength), with a corresponding increase in the equipment required. Furthermore, since the multiplexing occurs in the frequency domain rather than the time domain, this approach does not have stringent synchronization requirements and does not require memory buffers as would be the case with TDM approaches.
In addition, since the low-speed channels are combined in the frequency domain rather than the time or wavelength domain, this allows more flexibility in the types of low-speed channels which may be supported. For example, the low-speed channels (or the tributaries on which the low-speed channels are based) may be characterized by different data rates or different communications protocols so long as each low-speed channel does not exceed the frequency band allocated to it. As another example, each of the low-speed channels may be amplified or attenuated by different amounts in order to compensate for the specific transmission characteristics at that low-speed channel""s frequency band. Frequency bands with especially poor transmission characteristics may simply not be utilized. In contrast, TDM- or WDM-based approaches generally do not have these advantages.
The efficient conversion between optical high-speed channels and electrical low-speed channels also enables the use of an electrical ADM crosspoint to implement the ADM functionality of the FDM node. This yields further advantages since a crosspoint can be more flexible than other ADM solutions. For example, the crosspoint preferably can be configured to connect any input to any output. As a result, in addition to the basic add, drop, and pass-through functions, such a ADM crosspoint can implement any combinations of the above, including broadcasting or multicasting. This flexibility allows a single FDM node to be configured in a variety of ways to support a variety of network configurations. It also allows the FDM node to be easily reconfigured while in service. This facilitates the implementation of system reconfigurations with minimal disturbance to in-service traffic and also facilitates the implementation of fault-tolerance by enabling data streams to be efficiently re-routed to redundant hardware in the case of failure of the primary hardware.